This invention relates generally to signal generators and more particularly to signal generating systems for synthesizing low frequency triangular waveforms.
Certain known analog circuits for generating triangular waveforms, such as described by E. Harry Heflin, "Compact Function Generator with Enhanced Capability/Cost Ratio," pages 15-20 of the Hewlett-Packard Journal, July, 1973, rely on linearly charging and discharging a capacitor. By utilizing the principle of phase-locking a voltage controlled oscillator to an input frequency, such as the output of a digital frequency synthesizer, a very accurate frequency synthesis as well as compatibility with a digital input interface for frequency control can be obtained. Another known analog circuit for generating triangular waveforms involves direct switching of the capacitor charging current by the output of a frequency source, such as a digital frequency synthesizer, and a current source with an output proportional to the frequency of its input, such as a digital signal followed by a digital-analog converter and a voltage controlled current source. These prior art analog circuits suffer from many inherent disadvantages when used for generating low frequency triangular waveforms. To cover several decades of frequency generation, all of these methods require a multiplicity of range capacitors which must be switched electronically by expensive analog switches. For low frequency generation these capacitors become prohibitively large. Except for the phase-locked loop, the transfer functions of the current sources (typically voltage controlled) must be very linear, especially at low frequencies, to prevent unwanted changes in triangle amplitude as the input frequency is varied. At frequencies less than 100Hz the settling times of phase-locked loops become excessively long. In addition, the capacitors in the low pass loop filter portion of the phase-locked loop circuit become prohibitively large and expensive when designed for low frequency operation. In summary, no practical analog circuits are known which are capable of generating low frequency triangular waveforms with a high degree of accuracy in frequency and amplitude synthesis.
Certain known digital accumulator type triangular waveform generators such as those described in U.S. Pat. No. 3,633,017 entitled, DIGITAL WAVEFORM GENERATOR, issued on Jan. 4, 1972 to Arthur W. Crooke and in U.S. Pat. No. 3,654,450 entitled, DIGITAL SIGNAL GENERATOR SYNTHESIZER issued on Apr. 4, 1972 to Joseph A. Webb define a generalized method for generating any phase-dependent waveform and for performing frequency, phase and amplitude modulation on the basic waveform. However this method requires, in general, a digital clock, a digital accumulator or counter, and digital logic for mapping the phase into an output waveform. A specialized triangular waveform generator would not require the digital logic for mapping the phase into an output waveform but this would not substantially reduce the complexity of the method. While quite general and accurate, these digital logic accumulator circuits are very complex and expensive when used in the construction of a limited function, low cost waveform generator. In addition, digital logic accumulator circuits are limited in the generation of high frequency waveforms by the switching speeds of their logical components.